Field of the Invention
This invention relates in general to the construction of tubular members or pipes and in particular to a new and useful reinforced tube of plastic and to a method of manufacturing reinforced plastic tubes.
Reinforced tubes of plastic of known construction are mostly designed as so-called sandwich tubes comprising a layer of plastic-bonded granulated filler material, for example, sand, between glass fiber-reinforced layers of plastic, by an appropriate orientation of the glass fiber reinforcement in the respective layers, the axial and/or radial resistance of such a tube may be varied. Experience has shown that tubes having a great resistance both to radial and axial stresses are relatively difficult to manufacture and, in particular, need much glass fiber reinforcement whereby they become expensive. This leads to considerable difficulties, particularly in the construction of tubes of medium diameter (for example, between 40 and 150 cm) which are to be buried in the ground, on which, in addition, high tightness requirements are imposed.
The present invention is directed to a tube of plastic which is adapted to resist high stresses both in the axial and the radial direction. For this purpose, in accordance with the invention, a tube of plastic is provided with at least one helical reinforcing insert which is wound coaxially of the tube axis. This insert may be made of a fiber-reinforced plastic or a metal. It may be placed, for example, in an intermediate layer which is provided between two glass fiber-reinforced plastic layers or, for example, in a layer which is provided radially outside the usual reinforcing layers and, for example, further supports only a thin covering layer or an insulating layer.
The method of manufacturing such a tube, also constituting the subject matter of the present invention, comprises producing a fiber-reinforced plastic layer and providing this layer, during its formation at the earliest, with a helically wound reinforcement. Thus, for example, in a rotating mold for the tube, the helical reinforcing insert (helix) is inserted before or after a fiber-reinforced layer is built up, whereupon the material necessary for embedding the insert or forming a further layer is brought in. The helically wound reinforcing insert could, however, also subsequently be slipped over or wound on a tubular body which has been produced by centrifuging in a hollow mold or winding on a core, whereupon the tube would be finished by applying further material, for example, by winding or attaching shell pieces. It may be advantageous to introduce the helical insert only after producing an outer fiber-reinforced layer, but one may proceed also in a manner such that the fiber reinforcement of an outer layer is introduced simultaneously with the helical insert and the material necessary for the layer formation, i.e. plastic and filter (for example, sand) is brought in only subsequently.
In case such a tube is used for conveying liquids having a relatively high temperature, for example, hot water, difficulties are frequently met which are due to the unequal thermal expansion of a steel helix and a glass fiber-reinforced plastic. If the steel helix is placed in the middle zone of the tube wall, it may prevent a free expansion of the inner layer of the wall to such an extent that undesirably high stresses are produced between this inner layer and the freely expanding outer layer of the tube wall. Such stresses lead to a separation of the layers or a rupture, particularly in the presence of additional compressive and/or bending stresses such as occur in buried tubes. A placing of the steel helix in the outermost layer of the tube cannot solve this problem either since for example with the burying of such a tube, unavoidable bending or shearing stresses between the steel helix and the glass fiber-reinforced plastic may also lead to a rupture of the tube.
The present invention makes it possible to avoid even such difficulties by providing a tube, and a method for manufacturing such a tube, where stresses leading to a rupture cannot occur. For this purpose, the inventive tube of plastic comprises at least one helical insert which surrounds the inner layer of plastic and is enveloped by a protective layer which is separated from an outer plastic layer by a thermally insulating filler material.
The tube may be manufactured in a particularly advantageous manner so that the helix is wound on an inner tube of glass fiber-reinforced plastic and covered by a protective layer and that, thereupon, a jacket tube of plastic is positioned in coaxial and radially spaced relationship thereto and the annular space thus produced between the inner tube and the jacket tube is filled with a setting plastic foam.
Due to the insulating layer provided between the helix and the jacket tube, stresses leading to rupture can not occur and the tube thus obtained is excellently suitable, even in buried state, for conveying liquids having increased temperatures.
Accordingly it is an object of the invention to provide an improved pipe or tube construction which comprises an inner tubular first layer of glass fiber-reinforced plastic, an outer tubular second layer of glass fiber-reinforced plastic disposed around said first layer and spaced radially outwardly therefrom, and an intermediate layer between said first and second layers and containing plastic and granular filler material bonded together and to said first and second layers and further including a reinforcing insert embedded in the intermediate filler layer which comprises a continuous coil wound around said first layer.
A further object of the invention is to provide a method of forming a tube or pipe which comprises rotating a mold and adding a plastic and fiber material thereto to form an outer layer, adding a plastic and granular material to the first layer to bond it to the first layer and before it is through positioning a coil in the mold into the filler layer and continuing to form the filler layer over the coil to embed the coil in the filler layer and thereafter adding another layer of fiber reinforced material to the filler layer and bonding it thereto.
A further object of the invention is to provide a plurality of individual pipe sections having inner and outer reinforced plastic layers with an intermediate filler layer and a coil embedded in one of said layers which is wound around the axis of said layers, and wherein the innermost layer is arranged to have a length longer than the outermost and intermediate layers so as to abut against a neck section of a cylinder construction and including a sleeve part engaged around the overhanging ends of the innermost layers and being sealed thereto and having an outer wall part extending over the outermost layers and being sealed thereto.
A further object of the invention is to provide a pipe construction which is simple in design, rugged in construction and economical to manufacture.